1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser element, a vertical cavity surface emitting laser array element, a vertical cavity surface emitting laser device, a light source device, and an optical module.
2. Description of the Related Art
Vertical cavity surface emitting lasers (VCSEL: vertical cavity surface emitting laser is hereinafter referred to as “surface emitting laser element”) are used as light sources for various optical communications including optical interconnections, or other various devices for applications (for example, see Japanese Patent Application Laid-open No. 2005-252111). Because surface emitting laser elements emit laser beams vertically to substrates, a plurality of elements are able to be two-dimensionally arrayed on the same substrate more easily than conventional edge emitting laser elements. Moreover, they have many advantages because the volumes of active layers are very small, like laser emissions with extremely low threshold currents and low electric power consumptions being possible. Such a surface emitting laser element uses a distributed Bragg reflector (DBR) mirror, which is formed of a periodic structure of low-refractive-index layers and high-refractive-index layers and which is a mirror constituting a cavity.
When a DBR mirror is stacked on a substrate, there is a problem that dislocation occurs due to lattice mismatch between the substrate and the DBR mirror, and the surface emitting laser element becomes less reliable or defective. That is, because the layer thickness of the DBR mirror is thick, strain accumulates in the DBR mirror due to the lattice mismatch. As the DBR mirror becomes thicker, crystal defects aggregate and form a dislocation network. This is generally observed as linear dislocation (cross-hatches), and is a cause of a defect if introduced into an active layer of the surface emitting laser element. Particularly, cross-hatches extremely decrease the manufacturing yield for an array element. This is because the cross-hatches pass straight through each active layer of the array element.
To reduce this dislocation, conventionally, a surface emitting laser element that employs a substrate added with indium (In) to reduce a warp of the substrate and decrease the occurrence of dislocation (see, Japanese Patent Application Laid-open No. 2005-252111); and a surface emitting laser element, in which a lattice constant of a multi-element mixed crystal such as AlGaAsP or AlGaInP, which is a semiconductor material constituting a DBR mirror, is made close to that of a GaAs substrate to maintain lattice match and reduce dislocation (see, Japanese Patent Application Laid-open No. 2002-100834, and Japanese Patent Application Laid-open No. H6-196821), have been proposed.
Thermal impedances of DBR mirrors in surface emitting laser elements are desirably made low. Making the thermal impedance of a DBR mirror low have advantageous like its reliability being improved because of suppression of a temperature rise in the active layer; a roll-over current value being increased, which is advantageous when performing high frequency modulation; and the stability of emission wavelength being improved because of decrease in temperature dependency of reflected wavelength characteristics and the like.
As a method of making the thermal impedance of a DBR mirror low, there is a method of forming a DBR mirror with a semiconductor material having a high composition of aluminum (Al), which has high thermal conductivity, like AlGaAs having an Al composition of 0.8 or higher.
However, AlGaAs with a high Al composition has a lattice strain of approximately 0.14% for example, with respect to GaAs, and thus there is a problem that when the Al composition is increased in a DBR mirror made of AlGaAs formed on a GaAs substrate, cross-hatches generated on the surface are increased.
Moreover, conventional techniques for reducing the dislocation have the following problems. That is, in a technique of adding In to a substrate, it is technically very difficult to add In uniformly. In a technique of forming a DBR mirror with a multi-element mixed crystal material, it is not possible to make the Al composition high because of the multiple elements. In other words, it has been difficult to simultaneously make the thermal impedances and dislocation in BR mirrors low.